Since its introduction more than two decades ago, the traffic alert and collision avoidance system (TCAS) has provided tremendous safety advantages by reducing the risk of mid-air collisions. By interrogating the transponders of surrounding aircraft, TCAS analyzes the replies from these transponders to determine the range, bearing, and relative altitude (if reporting altitude) of each surrounding aircraft (“intruder”). TCAS provides a display for traffic awareness, and both aural and visual alerts are issued when a possible mid-air collision hazard becomes imminent. TCAS generally issues two levels of advisories. A traffic advisory (TA) is a caution level alert to inform the crew that closing traffic is in the vicinity of the crew's aircraft (“ownship”). If the intruder continues to close, TCAS issues a warning level resolution advisory (RA) that provides guidance to assure safe vertical separation.
Resolution advisories may further be categorized into two response types. A preventive RA requires the pilot to avoid certain deviations from the current vertical speed (e.g., “Monitor Vertical Speed”, RA requiring the pilot not to climb or descend while in level flight). A Preventive RA does not require the pilot to alter the airplane's existing flight path. A corrective RA requires the pilot to either deviate from current vertical speed (such as “Climb” RA while in level flight), or to maintain an existing climb or descent rate. TCAS provides a vertical speed target for the ownship to maneuver to or maintain in order to provide separation from the intruder.
TCAS bases the alerts on an assumed five-second crew reaction time to achieve adequate separation. A traffic display presents the location of intruder traffic. Compliance with a TCAS RA is mandatory unless the pilot considers it unsafe to do so, or if the pilot has better information about the cause of the RA and can maintain safe separation (e.g., visual acquisition of, and safe separation from, a nearby airplane, obvious TCAS system failure, etc.) The RA display is incorporated into the vertical speed indicator (VSI) or the primary flight display (PFD)/attitude direction indicator (ADI). Green and red zones are presented on the VSI or ADI (pitch ladder) which depict fly-to and avoidance zones, respectively. These are consistent with the required vertical rate or pitch attitude, or limitation of climb or descent, issued by TCAS to avoid a possible collision.
Operational experience shows that pilot response to a TCAS RA is at times incorrect or contrary to TCAS commands, for example the initial pilot response may be too slow, or may not be as aggressive as assumed, or may be in the wrong direction. It is also known that improper maneuvers may lead to excessive altitude gain/loss and incursion of adjacent flight levels. In order to alleviate these problems known in the art, various autopilot-coupled TCAS systems are currently under development and testing, which enable the autopilot to automatically fly TCAS RA maneuvers per the existing TCAS functionality. Upon full implementation, AP TCAS is expected to significantly reduce incorrect RA responses, resulting in an overall increase in safety.
As noted above, during a Preventive RA, the flight crew is not required to change the aircraft's flight path. Thus, safe separation from an intruder is assured on the current flight path. However, separation will be compromised if the aircraft is maneuvered into the “no fly” vertical speed area depicted on the VSI. Hence, the AP TCAS system does not maneuver the aircraft during Preventive RAs. However, a problem may arise when the aircraft's flight management system (FMS) is programmed with a vertical navigation (VNAV) path or other pitch mode manually selected on the mode control panel (MCP), and this path or mode is continued to be followed during a preventive RA, if the vertical speeds commanded approach the TCAS protected limits.
Accordingly, there remains a need in the art for improved TCAS systems, especially those that are coupled with an aircraft's autopilots systems. The improved TCAS systems would limit the autopilot from commanding vertical speeds contrary to TCAS commands during a preventive RA, and/or initiate an AP TCAS maneuver if the current vertical speed is nearing (or exceeds) the protected TCAS vertical speed limit. Both of these solutions would provide additional protection prior to any onset of the conventional corrective RA and minimize unnecessary maneuvering. Furthermore, other desirable features and characteristics of the disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.